In recent years, studies have been conducted on a system in which base station devices and wireless devices are connected by optical fibers by using a technology called, for example, Radio on Fiber (RoF). In RoF, wireless transmission signals are optically modulated by base station devices, are transmitted by optical fibers to wireless devices placed in remote site, are amplified by the wireless devices, and are then wirelessly transmitted. Accordingly, for example, it is possible to configure a system in which multiple wireless devices are connected to a single base station device, processes to be performed on signals are collectively performed by the base station device, and the signals are wirelessly transmitted from the respective wireless devices. Consequently, the overall cost of a wireless communication system can be reduced.
In general, when signals are sent by optical fibers in RoF, inter modulation distortion according to the frequency of the signals is generated. Namely, for a converter that converts, for example, electrical signals to optical signals, the range of electrical power in which signals can be converted while maintaining the linearity is relatively small and thus, if electrical signals outside this electrical power are converted to optical signals, inter modulation distortion is generated and thus the linearity of the signals is lost. Accordingly, there may be a case in which, after electrical power of a signal transmitted by an optical fiber is adjusted to a desired range, an electrical signal in which electrical power has been adjusted is converted to an optical signal and then this optical signal is transmitted by the optical fiber. Consequently, it is possible to decrease inter modulation distortion generated in the signal during the transmission performed by using the optical fiber and, as the result, it is possible to improve Spurious Free Dynamic Range (SFDR) that corresponds to a difference between the signal and the level of spurious.
Patent document 1: Japanese Laid-open Patent Publication No. 2014-103571
However, because noise generated in signals at the time of transmission performed by using optical fibers is not only due to inter modulation distortion, there is a problem in that an optimum signal to noise ratio (SNR) is not able to be obtained due to noise other than the inter modulation distortion. Specifically, in signal transmission using optical fibers, noise, such as thermal noise, shot noise of a photodiode, or relative intensity noise (RIN) of a light source. Unlike inter modulation distortion that is generated in a specific frequency band according to the frequency of a signal, these types of noise are added across a wide frequency band, resulting in a decrease in the SNR.
Due to such a decrease in the SNR, in a wireless communication system in which base station devices and wireless devices are connected by using optical fibers, the quality of transmission signals transmitted from, for example, a base station device to a wireless device may be degraded.